As transistors in integrated circuits scale to smaller and smaller dimensions, transistor performance is not keeping pace. One method to improve transistor performance is to apply stress to the transistor channel region to enhance carrier mobility. For example, tensile stress may be applied to the channel of an NMOS transistor to enhance electron mobility and compressive stress may be applied to the channel of a PMOS transistor to enhance hole mobility.
For strained device development and for the control of strain in manufacturing routine measurements of the local strain tensor in the channel region need to be made. Some of the more commonly used strain measurement methods include nano-beam diffraction (NBD), convergent beam electron diffraction (CBED), and geometric phase analysis (GPA).
A cross section of a PMOS transistor 1000 with the silicon in the source and drain areas 1004 and 1010 replaced with germanium doped silicon (SiGe) is shown in FIG. 1. Since germanium is a bigger atom than silicon, compressive strain is applied to the channel region 1002 under the transistor gate 1006. Stress enhancement techniques such as the deposition of overlying highly stressed films may be used to apply additional stress to the channel region.
In the callout 1008 in FIG. 2, the larger lattice constant of the SiGe in the source and drain areas 2006 and 2008 applies compressive stress in the transistor channel region 2004 forcing the silicon atoms to be closer together in this region than in an unstrained region such as the region 2002 below the channel.
A high-resolution, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of a PMOS transistor with SiGe source and drains is shown in FIG. 3. SiGe source and drains 3006 and 3008 are formed on either side of the transistor channel 3004 which underlies the transistor gate 3010. Single crystal silicon far below the transistor 3002 is essentially unstrained 3002 whereas silicon in the channel region 3004 may be highly strained.
As shown in FIGS. 4 and 5, in nano-beam diffraction (NBD) a collimated electron beam is scattered off the atoms in the sample 4006 forming a diffractogram such as in FIG. 5 after passing though objective lens 4004. The beam size of NBD may be in the range of 0.5 to 5 nm so the stress may be measured in the channel region of transistors with a channel length of about 5 nm or larger. Typically one diffractogram is taken in the strained channel region 3004 of a transistor and a second defractogram is taken in the unstrained single crystal region below the transistor channel 3002. The position of the diffraction points in the two diffractograms is compared. The amount of displacement of the points due to the compress stress may be related to the strain. Alternatively scan line data may be taken with many spots across the length of the channel and compared to an unstrained reference spot below the channel. In this way changes in the strain profile across the channel may be measured. NBD has a resolution of about 3 nm to 5 nm and a sensitivity for strain measurement of about 0.1%.
As shown in FIGS. 6 and 7, in convergent beam electron diffraction (CBED), a convergent beam of electrons 5010 is scattered off the atoms in the sample 5006 forming a diffractogram of disks as shown in FIG. 7, after passing through objective lens 5004. High-order Laue zone (HOLTZ) lines of the diffractogram of a strained region 3004 is compared with those of the diffractogram of an unstrained region 3002 to determine the strain. CBED has limitation in spatial resolution due to sample tilt to get proper HOZL lines, but has an increased sensitivity for strain measurement of about 0.01%.
Geometric phase analysis (GPA) is illustrated in FIGS. 8, 9, and 10. The diffractogram in FIG. 9 is a Fourier transformed image of the high resolution HAADF-STEM image in FIG. 8. Information regarding strain in various portions of the high-resolution HAADF-STEM image is contained in the blurring of the points such as g0 and g1 in FIG. 9. The various components of the strain tensor may be calculated from these points and used to form plots of the strain at various points in the HAADF-STEM image. An example plot of the xx component of the strain tensor is shown in FIG. 10. The light color in the channel region 1052 indicates compressive strain, the darker color in the source and drain areas, 1054 and 1056, indicates tensile strain, and the medium color under the channel region 1050 indicates a region of low to no strain. GPA has the advantage of simultaneously measuring strain throughout the entire HAADF-STEM image with a sensitivity for strain measurement of about 0.1% and resolution of about 2-4 nm.